FormsAuthentication.SetAuthCookie throwing NullReferenceException in async action

I am finding that FormsAuthentication.SetAuthCookie is throwing a NullReferenceException - Object reference not set to an instance of an object inside an async action on an azure website.

I found the following:

http://connect.microsoft.com/VisualStudio/feedback/details/743350/formsauthentication-setauthcookie-throws-nullreferenceexception-if-called-in-an-async-action-in-mvc4

However I already have

 <appSettings>
    <add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />

set and my code works fine locally I only experience the problem when I deploy to azure. I am using Azure Websites (https://www.windowsazure.com/en-us/home/scenarios/web-sites/), it was my understanding that this uses the web.config normally? I have also tried adding the appsetting through the Azure control panel

And adding .ConfigureAwait(false); to my awaited method but have had no luck.

The following code will throw the exception

public class TestController : Controller
    {
        public async Task<ActionResult> Index()
        {
            var httpResponse = await new HttpClient().GetAsync("http://www.google.com");
            FormsAuthentication.SetAuthCookie("test", true);
            return View();
        }
    }

Anyone know how I can get this to work?

UPDATE:

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
   System.Threading.Tasks.<>c__DisplayClass1c.<GetRethrowWithNoStackLossDelegate>b__1b(Task task) +91
   System.Threading.Tasks.TaskHelpersExtensions.ThrowIfFaulted(Task task) +15
   System.Web.Mvc.Async.TaskAsyncActionDescriptor.EndExecute(IAsyncResult asyncResult) +77
   System.Web.Mvc.Async.<>c__DisplayClass3f.<BeginInvokeAsynchronousActionMethod>b__3e(IAsyncResult asyncResult) +16
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +50
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +29
   System.Web.Mvc.Async.<>c__DisplayClass39.<BeginInvokeActionMethodWithFilters>b__33() +59
   System.Web.Mvc.Async.<>c__DisplayClass4f.<InvokeActionMethodFilterAsynchronously>b__49() +240
   System.Web.Mvc.Async.<>c__DisplayClass37.<BeginInvokeActionMethodWithFilters>b__36(IAsyncResult asyncResult) +12
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +50
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +31
   System.Web.Mvc.Async.<>c__DisplayClass2a.<BeginInvokeAction>b__20() +23
   System.Web.Mvc.Async.<>c__DisplayClass25.<BeginInvokeAction>b__22(IAsyncResult asyncResult) +128
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +50
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +26
   System.Web.Mvc.<>c__DisplayClass1d.<BeginExecuteCore>b__18(IAsyncResult asyncResult) +14
   System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +25
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +55
   System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +41
   System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +25
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +55
   System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +28
   System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10
   System.Web.Mvc.<>c__DisplayClassb.<BeginProcessRequest>b__4(IAsyncResult asyncResult) +28
   System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +25
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +55
   System.Web.Mvc.<>c__DisplayClasse.<EndProcessRequest>b__d() +31
   System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f) +7
   System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) +23
   System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +59
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
   System.Web.CallHandlerExecutionStep.OnAsyncHandlerCompletion(IAsyncResult ar) +96

UPDATE

I found it works if setting the Cookie manually

FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(1, "test", DateTime.Now, DateTime.Now.AddMinutes(30), true, null, FormsAuthentication.FormsCookiePath);
            string encTicket = FormsAuthentication.Encrypt(ticket);
            Response.Cookies.Add(new HttpCookie(FormsAuthentication.FormsCookieName, encTicket));

I'm not going to answer the question though as would like to know why FormsAuthentication.SetAuthCookie is throwing the exception and why it is behaving differently on Azure Websites